The present invention relates to an optical radar for measuring and inspecting a distance to a target measurement object and its shape and, more particularly, to an optical radar having an optical surface sensor and capable of obtaining a light return time difference as a phase change amount in signal processing, thereby obtaining distance/shape data at high speed.
An optical radar is a technique for measuring a change in propagation time until emitted light becomes incident on a measurement target and reaches a sensor, by directly measuring the propagation time of one optical pulse or by indirectly measuring propagation times as phase change amounts of an optical pulse train emitted and reaching a sensor, and for obtaining a change in distance to the measurement object or its shape in accordance with a propagation time difference.
A conventional optical radar represented by a laser radar has been used to measure the shapes of a construction (e.g., a dam) and an airplane wing. Along with the recent advance in high-frequency techniques and optoelectronic techniques, optical radars have also been used to measure the shapes of nearer objects.
The state-of-the-art optical radar light-receiving portion as the main feature of the present invention will be briefly described below.
An optical radar requires some high-speed light-receiving element for detecting and measuring arrival of sensing light. Unlike in a CCD camera, a high-speed photodiode and a photo multiplier, which have been conventionally popular, cannot be two-dimensionally arranged in the state of the art while maintaining high-speed characteristics. For this reason, according to a conventional implementation, as shown in FIG. 1, at least one of irradiated and reflected beams is scanned with a rotary mirror to measure the shape of an object using one light-receiving element.
Some optical surface sensors are still used at present and will be compared with each other below.
Conventional popular surface-light-receiving type methods of measuring changes in light amounts are represented by the following typical examples.
1) Television camera PA1 2) Mechanical scanning by an optical sensor PA1 3) Image dissector (electron tube random access camera) PA1 4) Streak camera
In the above examples, the television camera 1) is the most popular and lowest in cost. However, the intensity of a certain pixel cannot be sampled at the frame rate (1/30 sec in the NTSC scheme) or faster. For this reason, it is impossible to measure the phase of reflected light of a laser radar which generally changes at about 1 GHz.
In recent years, even in a CCD with the most advanced shutter function, it is often difficult to measure a change in light amount of a frequency largely exceeding MHz due to the sensitivity limitations.
The mechanical scanning 2) is two-dimensional scanning of a high-speed photodiode using a moving stage. With this arrangement, the distribution of return time differences of surface beams can be relatively easily measured. It is still difficult to realize high-speed scanning. Therefore, this mechanical scanning has a problem on simultaneity in measurement.
The random access camera 3) is used in a heterodyne interferometer for measuring modulated light of about 100 kHz and has achieved good results. At present, however, the random access camera cannot cope with a modulation frequency on the order of about GHz. In addition, since the random access camera continuously repeats point measurements, the phases of points within the entire field angle cannot be simultaneously measured, and errors tend to occur with a lapse of time.
The streak camera 4) is suitable for recording a change in ultra-high-speed phenomenon as a function of time. This camera can naturally be used in distance measurement using light return time differences. However, a considerably large number of two-dimensional images must be obtained to obtain light return times within the entire field angle. To obtain a change in distance on the order of about mm, a time-resolving power of about 1 ps is required because the streak camera employs real-time analysis. At present, the streak camera results in a very expensive measuring system.
As described above, it is impossible for the conventional optical surface sensors to measure, with a high S/N ratio, reflected light whose intensity changes at high speed exceeding about 100 MHz, and to determine the phase at high speed with high precision.
The present applicant proposed a distance measuring method and apparatus using an image pickup means having a variable light amount/sensitivity function represented by an image intensifier with a gate, as disclosed in Japanese Patent Laid-Open No. 6-109418 entitled "Distance Measuring Method and Apparatus" (laid-open date: Apr. 19, 1994). The present invention is a proposal associated with the signal processing method in this former invention.
The present invention uses a sensor which is adapted to measure the modulation phase of a finitely spread, modulated two-dimensional beam of an optical radar without mechanical scanning. First of all, a general principle of phase measurement will be described below.
Assume that a target measurement signal is represented by a function f(t), and that the intensity and phase of one of the frequency components mixed in this signal are to be obtained. If a function f(t) consisting of a large number of angular frequency components .omega. has a normal signal form represented by a Fourier transforms this function is represented as follows: ##EQU1## and its coefficients are defined as follows: ##EQU2##
If two reference signals cos.omega..sub.0 t and sin.omega..sub.0 t are prepared to measure the phase of a frequency component .omega..sub.0, and the following equations are obtained by a certain means: EQU PC=A(.omega..sub.0)=.intg.f(t).times.cos .omega..sub.0 tdt EQU PS=B(.omega..sub.0)=.intg.f(t).times.sin .omega..sub.0 tdt
the independent frequency components are eliminated, and a coefficient representing the ratio of the frequency component .omega..sub.0 in the function f(t) is obtained.
If phase "0" is given for the reference signal sin.omega..sub.0 t=0, a phase .theta. of the frequency component .omega..sub.0 is represented using PC and PS as follows: EQU .theta.=tan.sup.-1 (PS/PC)
The phase .theta. is represented as defined above to ignore a normalization constant by using a measurement value expressed in the form of PS/PC.
Note that a normalization value P for a measurement unit is expressed as follows: ##EQU3## so that the phase can also be obtained as follows: EQU .theta.=sin.sup.-1 (PS/P)
If this value is represented by a voltage in units of V, P is equivalent to the effective value of the frequency component .omega..sub.0.
Note that, in actually calculating arctangent and arcsine values, they must be determined under conditions in consideration of their definition and value ranges.
This method is used in the field of electronic and communication fields for a network analyzer, a lock-in amplifier, a measuring instrument (e.g., a phase meter or a wow and flutter meter), and various detectors. This method is one of the most fundamental principle of phase measurement. A circuit arrangement which employs this principle of phase measurement is shown in FIG. 2.
In an optical radar, using the phase obtained as described above, a distance difference x is defined as follows: EQU x=.theta..multidot.c/(2.pi.f)
(x=.theta..multidot.c/4.pi.f for reciprocal measurement) where .theta. is the resultant phase difference, f is the modulation frequency, and c is the velocity of light.
If a signal is an electrical signal which changes as a function of time, the above multiplications can be easily achieved using a mixer, a switch, and the like.
It is, however, very difficult to perform multiplication for an object having a two-dimensional spread, such as an image.
For example, to obtain a resolution of 512.times.512 pixels like a television image, 250,000 or more mixers and switches are required. It is not easy to realize this circuit arrangement using even the current LSI techniques.
Modulation at 100 MHz or more is required to perform detection in an optical radar. It is difficult to synchronously drive a large number of switches at a high frequency because of the limitations of circuit constants.
As described above, the conventional optical surface sensor cannot be used in an optical radar.